A "chase" is an artwork designated by its creator as special in some way. It may be extremely limited or different somehow. It is represented by this rarity symbol:
A chase artwork does not show up as an empty slot in your collection if you do not own it, so you may not even know it existed when you come across it. Do not assume chase artworks within the same collection are equally scarce. The best way to find the print count of a chase artwork is to look at its detail page.
The following chase artwork, for example, has 500 prints total: